1. Field of the Invention
The present invention relates to an LED array which is used for instance in an illumination system for a read-out photosensor, and more particularly it relates to an LED array divided into blocks which may independently be illuminated or extinguished and to a partitive driving method for such an LED array.
2. Description of the Prior Art
In general, an image pick up device such as facsimile reads out image information of an original image by radiating light rays from a light source upon the original image and by detecting the reflected light therefrom with a photosensor. Among photosensors of this kind, there is known in the art a contact type photosensor which is mounted on an original read-out section of such a device. The original read out section for reading out an original includes: photosensors facing the original and having a one dimensional area corresponding to the width and length of the original; a one dimensional light source; and a light transmission member such as CELFOC lens (merchandise name, manufactured by Japan CELFOC Kabushiki Kaisha) and optical fibers. The one dimensional light source for use with the contact type photosensor is for example a fluorescent lamp, or an array member in which LED chips are disposed linearly at constant intervals (such as 2.5 mm).
The use of an LED array as a light source, however, gives rise to a problem that the accumulation of heat generated in the LEDs causes a temperature rise in the LED array thus reducing the life span of the device. This is because a plurality of LEDs on the LED array have heretofore been illuminated concurrently. To avoid a temperature rise, it can be thought of that a larger radiator or an air-cooling fan may be provided for radiating heat. However, these countermeasures can not settle the problem entirely satisfactorily in that they become a bar to the miniaturization of the original read out section and a rise in cost is brought about.